The present invention relates to a mass spectrometer system that includes an atmospheric pressure ionization interface suited for use in combination with a liquid chromatograph, i.e., as a liquid chromatograph mass spectrometer.
A mass spectrometer (hereinafter referred to as “MS”) is occasionally used in combination with a liquid chromatograph, as a liquid chromatograph mass spectrometer (hereinafter referred to as LC/MS). In an LC/MS, the components of a sample separated by liquid chromatography are introduced into the MS for mass spectrometry. In order to perform mass spectrometry, an interface is required to ionize the separated components. In recent years, a method of performing ionization under atmospheric pressure, such as an electrospray interface or an atmospheric pressure chemical ionization interface, has been generally employed as the interface for an LC/MS.
The mass spectrometer located downstream from such an interface is generally operated under a high vacuum condition. Accordingly, an LC/MS employing the atmospheric pressure ionization method usually comprises an atmospheric pressure ionization chamber for ionizing the liquid introduced from the liquid chromatograph unit under atmospheric pressure, and an intermediate evacuation chamber disposed between that and the mass spectrometry chamber with a built-in mass spectrometer. Evacuation systems are disposed in the intermediate evacuation chamber and the higher vacuum evacuation chamber downstream therefrom so that the degree of vacuum increases gradually from the upstream to the downstream chambers.
FIG. 3 is a schematic view of one example of such a conventional LC/MS.
In the figure, reference numeral 1 denotes a liquid chromatograph unit, 20 denotes a mass spectrometry unit, and 10 denotes an interface unit that couples the two.
The liquid eluting from the liquid chromatograph unit 1 is atomized in the interface unit 10 and sprayed from a nozzle 14 into the atmospheric pressure ionization chamber 15 where the sample component molecules contained in the elution are ionized. The ions generated are led through a capillary tube 11 to a roughly evacuated intermediate evacuation chamber 21, where the ions are converged by a convergent lens 24, and sent to a higher vacuum second intermediate evacuation chamber 22, where they are converted into a beam by an ion lens 25.
The ions are then introduced into the mass spectrometry chamber 23, which is maintained under an even higher vacuum, and sent to a central space in a quadrupole filter 26 composed of four rod electrodes. A voltage, with an AC voltage superimposed on a DC voltage, is applied to the quadrupole filter 26. Only the ions having a specific mass number (more precisely, mass-to-charge ratio) that corresponds to the voltage pass through the quadrupole filter 26 and reach the ion detector 27. At the ion detector 27, the current in correspondence with the number of ions reached is taken out as an output signal.
The capillary tube 11 is disposed through the partition wall 16, which separates the atmospheric pressure ionization chamber 15 from the intermediate evacuation chamber 21, so that the atmospheric pressure ionization chamber 15 is in communication with the intermediate evacuation chamber 21 only through the capillary tube 11. Accordingly, a portion of the gas present in the atmospheric pressure ionization chamber 15 flows through the capillary tube 11 into the evacuated intermediate evacuation chamber 21.
The capillary tube 11 constitutes a desolvating unit 12 in conjunction with a heating block 12a fitted around the tube. The desolvating unit 12 functions as a means for removing solvent components contained in the charged particles generated in the atmospheric pressure ionization chamber 15. In other words, a portion of the charged particles sprayed from the nozzle 14 is caused to flow into the capillary tube 11 due to the pressure difference between the atmospheric pressure ionization chamber 15 and the intermediate evacuation chamber 21, and is heated by the heating block 12a, thereby promoting the desolvating process as the particles are introduced into the intermediate evacuation chamber 21.
Non-volatile components of the samples analyzed or inorganic salts from the liquid mobile phases used can accumulate inside the capillary tube 11. Thus, the capillary tube requires periodic maintenance that entails removal for cleaning or replacement. In order to reduce the down time for carrying out such maintenance, there has been made available a system, which includes an isolation gate 13 disposed on the partition wall 16, as indicated by the broken line in FIG. 3, to allow for the removal of the capillary tube 11 without lowering the degree of vacuum. An isolation gate is generally an opening that is formed for placing or removing an object through a partition wall of a vacuum chamber. In this instance, the capillary tube 11 is detachably inserted through the isolation gate 13, which is constructed so as to automatically close the opening when the capillary tube 11 is pulled out.
FIG. 4 illustrates one example of such a conventional self-closing isolation gate 13. In FIG. 4, the isolation gate 13 is constructed integrally with the heating block 12a, and the left side of the partition wall 16 is maintained at atmospheric pressure and the right side is maintained at an approximate vacuum. The capillary tube 11 is inserted through the heating block 12. In the state shown, the ball 33 is pushed up by the capillary tube 11. When the capillary tube 11 is pulled out to the left, the ball 33 is dropped by the force of the spring 34 so as to block the hole created after the capillary tube 11 is removed. See, for example, the disclosure of Japanese Laid-open Patent Publication No. 2002-198006.
Reference numeral 35 denotes a cover that prevents contamination from the particles being sprayed in the atmospheric pressure ionization chamber 15.
Such an isolation gate 13, as one example thereof is shown in FIG. 4, is a type of valve mechanism, and its complex construction can cause various problems. In addition, it has the shortcoming of reduced efficiency in transporting ions, since ions are dispersed in this complex valve portion. As shown in FIG. 3, moreover, the convergent lens 24 is often placed in the intermediate evacuation chamber 21 where the isolation gate 13 is located. Thus, the isolation gate 13 restricts the positioning of the convergent lens 24.
The present invention has been developed in view of the aforementioned problems. It is an object of this invention to provide an atmospheric pressure ionization MS with improved maintainability and utilization by omitting the isolation gate, and to improve the construction so as to enable the installation and removal of the capillary tube without breaching the vacuum.
Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.